Rotary atomising heads for electrostatic spray guns



April 13, 1965 M, A R, POINT 3,178,114

ROTARY ATOMISING HEADS FOR ELECTROSTATIC SPRAY GUNS Filed 001;. 25, 1961 2 Sheets-Sheet l DIRECT 9 I use: 1 u

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HIGH 9 VOLTAGE SOURCE 7 men mass 9 VOLTAGE F; Inventor Md @Avfm Attorney;

A ril 13, 1965 M. A. R. POINT 3,178,114

ROTARY ATOMISING HEADS FOR ELECTROSTATIC SPRAY GUNS Filed 0012. 25, 1961 2 Sheets-Sheet 2 -"f 2/ 13 & HIGH DIRECT VOLTAGE SOURCE Inventor A Niamey;

United States Patent ROTARY ATOMISiNG HEADS FOR ELECTRG- STATIC SPRAY GUNS Marcel Auguste Roger Point, Grenoble, France, assignor to Societe Anonyme de Machines Electrostatiques, Grenoble, France, a French body corporate Filed Oct. 25, 1961, Ser. No. 147,574 Claims priority, application France, Nov. 2, 196i), 842,739; Dec. 2., 1%0, 845,773 4 Claims. (Cl. 239-15) It is known that a rotary atomisation head made of conductive material may sometimes have drawbacks in circumstances, for instance, in which the risk would be run of accidentally bringing it to within a short distance from an object having a great difference of potential from the head. For this reason the applicant has had the idea of making such heads of a material which is insulating or only slightly conductive.

Up to now, however, manufacturing difiiculties are met with in the case of certain high resistivity materials, otherwise known as semi-conducting materials, having a resistivity range between and 10 ohm cmF/cm. (hereinafter referred to as high resistivity materials). Thus, for example, with thermosetting resins of the ethoxyline (or epoxides) type, containing carbon black, it has been found that the sharp edge of these heads where the atomisation and projection of the paint particles takes place is very fragile. Experience has shown that such sharp edges deteriorate rapidly and thus cause the electrostatic head to function badly.

Numerous tests have also shown the difficulty of finding a high resistivity material having a good mechanical resistance allied to a high resistance to the solvents used in paints and other covering materials.

A good degree of mechanical resistance may however be found in certain rubbers, but it has been found in this case that the simple impregnation of solvents fairly quickly brings about a swelling of the rubber which shows itself in intolerable distortions of the sharp edge of the head. I

The present invention therefore has as an object an improved construction of a rotary electrostatic head allowing the aforementioned drawbacks to be avoided.

It is particularly characterised in that the part of the rotary head comprising the sharp edge for centrifugal atomisation and at which the covering material, e.g. paint, arrives is made of an insulating material, that is to say a material whose resistivity exceeds 10 ohm cm. cm., and in that a portion made of a high resistivity material has a sharp edge which is located in the immediate neighbourhood of the edge of the insulating portion, this high resistivity portion being connected to a pole of a source of high direct voltage.

It may thus be seen that with such an improved construction, the centrifugal atomisation of the covering material and the creation of the electrostatic field operations are effected separately by an insulating portion and by a high resistivity portion respectively, both being rotatably driven at high speed, the two portions being provided with co-operating sharp edges.

In the case in which a high resistivity material is used to constitute the portion allowing the electrostatic field to be created, one profits from the advantage of a diminution in the energy of any accidental sparks which could be produced if the electrostatic head is brought close to an object at a very different potential, this high resistivity portion then acting as a damping resistance.

This resistance may be measured by placing said portion between two electrodes connected to the terminals of a sourceof high direct voltage providing a constant current of 0.2 mm, for example.

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So that the evaporated solvents may be prevented from inflaming it is necessary that, under the above conditions, the voitage at the terminals be at least 2,000 v., which corresponds to a minimum resistivity of about 10 ohmcm. for said high resistivity material.

In order better to show the technical characteristics and advantages of the invention, several embodiments thereof will be described, it being understood that these examples have no limitative character as regards the methods of operating the invention and the applications to which it may be put.

FIGURE 1 shows a section through an electrostatic head according to the invention mounted on the end of a gun, the end of the barrel of which has been diagrammatically shown, as has the axis of revolution and the paint feed pipe.

FIGURES 2 and 3 are similar views but relate to modifications which are also in accordance with the invention.

FIGURES 4 and 5 respectively show front and sectional views of a head according to the invention provided with fins.

FIGURE 6 diagrammatically shows the fixing of the fins, only two of which have been shown.

Referring to FIGURE 1, at 1 may be seen the outer insulating portion of the head, which is substantially cylindrical, comprising the sharp edge 1' at which the paint arrives, delivered to the head through the pipe 5 located in the barrel 4 of the gun. This feed pipe in no way interferes with the rotation of the head.

The revolving hub 2 of the head which is integral with the insulating revolving shaft but which is conductively connected at the end 6 to a source 9 of high direct voltage is also conductively connected through the spring 10 to a member 3 of high resistivity material having the general shape of a cone which narrows down and finally ends at 3' in an edge at a short distance from the edge 1' of the outer insulating portion 1. In this manner the paint, forced by centrifugal force against the inner wall of this insulating portion 1 does not normally come into contact with the high resistivity member 3.

Thus it'may be seen that the paint (and the solvent contained therein) is prevented from contacting the high resistivity member 3. Furthermore the proximity and the relative arrangement of the sharp edges of the members 1 and 3 allow the electric field to be created at the actual place at which the centrifugal atomisation of the paint is produced, at the end of the sharp edge of the member 1. This latter also mechanically protects the high resistivity member 3.

Experience has shown that the improved heads according to the invention give excellent results from all points of view.

In the modification which will be described below, the same reference numerals indicate the members having the same function as in FIGURE 1.

Referring firstly to FIGURE 2, it may be seen that the centrifugal atomising insulating member 1 which has a finely pointed conical shape is arranged inside the highly resistivity member 3 which has a generally cylindrical shape and the ionising sharp edge 3 of which, which creates the electric field, is located near the outside of the sharp edge 1'.

Preferably the insulating edge 1' projects slightly in,

3 of which is connected to said object, which is generally earthed. It will be noted in this embodiment that only the centrifugal atomisation insulating member 1 rotatablydrive'n.

The paint arriving at passes through the openings 8, along the inner terminal surface of the insulating member 1 and, forced by centrifugal force, arrives at the edge 1. coming' into contact with the high resistivity member '3.

FIGURE 3 shows another embodiment according to the invention and in which the attachment of the centrifu'g'al at'orn'isingi insulating member 1 is differently made.

It may in fact be seen that this member '1 is made in the shape of a sharp-edge ring mounted directly on an inner circular edge of the high resistivity member 3 the hub Z of whichmay be either of high resistivity or conducting material and is fixed to the electrically conducting end6 of the r'evolvirigsh'aft' of the gun. This end is of course connected by an suitable means to one of theterm'inals of a source 9' of high direct voltage the other terminal of which is connected to the object to be covered, generally earthe'd.

In this einb'odir'nent'the paint passes through the openings 8 of the high resistivity member 3, over the inner surface ofthi's member (without touching its sharp edge 3'), then over the inner surface of the'in'sulating membar '1' and arrives at it's centrifugal atomising sharp edge 1'. v I

In this embodiment thetwo members 1 and 3 revolve at hi'gh speed at the same time, asin the case of the embodiment in FIGURE 1.

Those skilled in the art will easily understand that many alterations in detail may be made to the foregoing embodiments without departing from the scope of the present invention. 7

Thus, for instance, the high resistivity member 3 of FIGURES 1, 2"and 3 could be 'made, if for any reason it" were desired, of a good electrically conducting material e'.g'. 'met'al, its role being to create the electric Theheads according to the invention are also suitable for'the addition of fins whichhave the purpose of creating an axially-directed draught directed toward the object to be" covered; this draught increasesthe safety of the gun when in use due to the turbulence of the air causedinthe zone in the immediate vicinity of the ator'nisation' zorie. In fact the ratio of the quantity of air in relation to that of the fuel (evaporated solvent) increases so that the probability of combustion decreases.

Referring to FIGURES 4, 5 and 6, relating to this last embodiment, it may be seen that the insulating member 1, made for example of nylon, is fixed by any suitable means on to a hollow metal socket screwed on the metal end 6' of the shaft providing the high-speed rotation of-the electrostatic head. This insulating member [is provided on its outer surface with a number of fins 12 only three of which have been shown on FIGURE 4, but which are also angularly distributed over the entire perimeter ofthe head like those of a fan.

on the other hand the high resistivity member 3 with the sharp edge 3 is secured concentrically inside the insulating member 1 so that the sharp edge 3 is located in the neighbourhood of the insulating sharp edgel.

The fixing of this high resistivity member 3 is effected by means of a preferably insulating screw 13, of nylon for example, which is screwed into the hollow electrically conducting socket 1'1 and the head 14 of which ensures the gripping of the hub of this member 3'against the terminal surface of the socket 11.

It may thus be seen that the high electrical voltage applied-to the conductive end 6 of the shaft from the It does not, therefore, run the risk of source 9 is also applied to the socket 11 and the member 3 which is gripped against this socket.

Here again the operations of centrifugal atomisation (the insulating member 1) and of the creation of the electrical field (the high resistivity member 3) have been separated, as in the preceding embodiments.

I claim:

1. In an apparatus for electrostatic and centrifugal atomisation of a fluid coating material, a spraying head 'cornprising'a first rotatable member of insulating material' having a peripheral sharp edge, a second member of high resistivity material arranged coaxially around the said first member and having a peripheral sharp edge, the sharp edges of the 'said'first and second members being adjacent to one another, but spaced a short distance apart, means for supplying fluid coating material so as to form a film on at least that part of said "first rotatable member adjacent to its sharp edge, a source of high direct current voltage and means for connecting said source of high voltage to the said second member.

2. In an apparatus as claimed in claim 1' wherein the second member has a substantially cylindrical form and said first member is partially the shape of a cone, a rotatable shaft, and means for attaching the first member to one end of the rotatable shaft. 7

3. In an apparatus for electrostatic and centrifugal atomisation of a fluid coating material, a spraying head comprising a first substantially cylindrical rotatable member 'of insulating material having a peripheral sharp edge, a second member of generally conical form made of highly resistivity materialarranged coaXially within the said-"first member and having a peripheral sharp edge, the sharp edges of the said first and second members being adjacentto one another, but spaced a short distance apart, a rotatable shaft having at least its terminal part made of conducting material, a hub member I made of conducting materialfitted to said shaft, means conne'cting both sai'd first and secondmembers to said hub meinher, means for suppling fiuid coating material so as to form a film on at least that part of sa'idfirst rotatablelmember adjacent to its sharp edge, a source o f'hi gh direct- -currer1t voltage and means connecting the said source of high voltage to the said terminal part of said shaft.

4. In an apparatus for'elec'trostatic and centrifugal atomisation of a fluid coating material, a spraying head comprising a first rotatable member of insulating material having a peripheral sharp edge, a second member of generally cylindrical form made of high resistivity material arranged coaxially around the said first member and having'a peripheral sharp edge, said first member being carried by saidsecond member and the sharp edges of the said first and second members being adjacent-to one another, but spaced at short distance apart, a rotatable shaft, means for mounting said second member on the rotatable shaft, means for suppling fluid coating material so as toform a film on at least that part of said first rotatable member adjacent to its sharp edge, a source of high direct current voltage and means for connecting said source of high voltage to the said second a member.

References Cited in the file of this patent UNITED STATES PATENTS 3,082,956 Point Mar. 26, 

1. IN AN APPARATUS FOR ELECTROSTATIC AND CENTRIFUGAL ATOMISATION OF A FLUID COATING MATERIAL, A SPRAYING HEAD COMPRISING A FIRST ROTATABLE MEMBER OF INSULATING MATERIAL HAVING A PERIPHERAL SHARP EDGE, A SECOND MEMBER OF HIGH RESISTIVITY MATERIAL ARRANGED COAXIALLY AROUND THE SAID FIRST MEMBER AND HAVING A PERIPHERAL SHARP EDGE, THE SHARP EDGES OF THE SAID FIRST AND SECOND MEMBERS BEING ADJACENT TO ONE ANOTHER, BUT SPACED A SHORT DISTANCE APART, MEANS FOR SUPPLYING FLUID COATING MATERIAL SO AS TO FORM A FILM ON AT LEAST THAT PART OF SAID FIRST ROTATABLE MEMBER ADJACENT TO ITS SHARP EDGE, A SOURCE OF HIGH DIRECT CURRENT VOLTAGE AND MEANS FOR CONNECTING SAID SOURCE OF HIGH VOLTAGE TO THE SAID SECOND MEMBER. 